Particle Size Estimation and Monitoring in a Bubbling Fluidized Bed Using Pressure Fluctuation Measurements

Transcription

1 Refereed Proceedings The 2th International Conference on Fluidization - New Horizons in Fluidization Engineering Engineering Conferences International Year 2007 Particle Size Estimation and Monitoring in a Bubbling Fluidized Bed Using Pressure Fluctuation Measurements Clive Davies Donal Krouse Alison Carroll Massey University, C.Davies@massey.ac.nz Industrial Research Limited, d.krouse@irl.cri.nz Taupo This paper is posted at ECI Digital Archives. xii/56

2 FLUIDIZATION XII 465 Davies et al.: Particle Size Estimation and Monitoring in a Bubbling Fluid Bed PARTICLE SIZE ESTIMATION AND MONITORING IN A BUBBLING FLUIDIZED BED USING PRESSURE FLUCTUATION MEASUREMENTS Clive E Davies, Institute of Technology and Engineering, Massey University, Private Bag 222, Palmerston North 4442, New Zealand T: ; F ; E: C.Davies@massey.ac.nz Donal Krouse, Industrial Research Limited, P. O. Box 3-30, Lower Hutt, New Zealand Alison Carroll, Taupo, New Zealand ABSTRACT Pressure time data for binary mixtures of silica sand have been examined with particular reference to correlation between mean particle size, standard deviation and parameters obtained through attractor reconstruction techniques. The ratio of total variance to high speed variance is a particularly useful parameter for regime identification. INTRODUCTION At superficial velocities greater than the minimum fluidizing velocity, U mf, the amplitude of fluctuations in the pressure drop across a bubbling fluidized bed increases as superficial velocity increases and is proportional to the difference between superficial velocity, U, and minimum fluidizing velocity (). Puncochar et al (2) have used the apparent linear proportionality between bed pressure drop standard deviation, σ p, and U as the basis of a method for determining U mf but recommend the method be restricted to Re p <30 and U<2.5 U mf, as outside these bounds the relationship between σ p and U was non-linear. Davies and Fenton (3) showed that σ p measurements could potentially provide a means for direct monitoring of mean particle size in a bubbling bed, and Davies et al (4) estimated mean particle size using an expression that follows from the premise that σ p is linearly proportional to (U U mf ) using a data set for which this condition was satisfied. However they reported that estimates of U mf obtained from σ p measurements, U mfσ, were larger than, U mf P, obtained from the Ergun equation and bed pressure drop measurements (5) and also noted that below U mfσ, σ p, though very small, rose to a maximum value before decreasing and then increasing rapidly as an apparently linear function of U. This observation in particular and the difference between the values of U mfσ and U mf P has prompted us to re-examine the data used by Davies et al (4). Published by ECI Digital Archives, 2007

3 466 DAVIES, KROUSE, TAUPO EXPERIMENTAL The 2th International DATA Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 56 [2007] The data previously measured by Davies et al (4) were obtained by fluidizing six batches of silica sand, particle density 2480 kg m -3, prepared from reference samples termed Coarse and Very Fine; the size distributions are shown in Figure. Bed pressure drop was logged at 50 Hz and, in some cases, 200 Hz for 0 (U / U mf )<~5. Minimum fluidizing velocities and the proportions of the six mixtures are shown below in Table. Details of the fluidized bed, diameter 00 mm, have been previously given by Davies and Fenton (3). Pressure fluctuations are resolved to ~ Pa using a Motorola MPX0DP pressure transducer and data acquisition unit, calibrated using a manometer. Figure 2 is a typical plot of σ p versus U and is for Mixture 4. Semi-logarithmic axes are used to emphasize Weight % undersize Particle size (microns) Figure Size distributions of Coarse and Very Fine sand Very Fine Coarse the non-zero values of σ p over the whole range of U. Standard deviation, σ p, is small but non-zero for U < U mfσ and appears to pass through a maximum before decreasing and then rapidly increasing linearly with U at U > U mfσ. The trends shown in Figure 2 for U < U mfσ were consistent and reproducible, and a plot of σ p versus particle diameter, for a constant U, despite some scatter, suggests a functional relationship between these variables as can be seen in Figure 3, which is for a superficial velocity of ms -. It thus appears that σ p is affected by bed structure Table Properties of test materials Material Very Fine Coarse U mf P U mfσ weight % weight % ms - ms - Coarse V. Fine Mixture Mixture Mixture Mixture Mixture * Mixture * * insufficient data for reliable estimate below the minimum fluidizing velocity as well as above it. This notion is qualitatively supported by the form of the pressure drop time-series plots. Figure 4 shows pressure drop: time traces for Mixture 4 for six superficial velocities spanning the range from (U / U mf P ) 0 to (U / U mf P ) 5. Each plot shows normalized data sampled at 50Hz over a period of ~60 seconds; note that σ p is given to indicate the relative vertical scale which is different in each pressure trace. 2

4 FLUIDIZATION XII 467 Standard deviation (Pa) Davies et al.: Particle Size Estimation and Monitoring in a Bubbling Fluid Bed U mf P U mfσ Superficial velocity (m/s) Standard deviation [Pa] Figure 2 Standard deviation of bed pressure drop as a function of superficial velocity; Mixture standard deviation 3.5 pressure drop Mean diameter [µm] Bed pressure drop [Pa] ANALYSIS Figure 3 Standard deviation and bed pressure drop as a function of particle diameter; U=0.008 m s - Variance onset of fluidization The total variance of a sample, V TOT and the high frequency variance (6) of a sample, V HI, are respectively given by Equation () and Equation (2): V 2 ( xi x) i= n TOT = () ( n ) Published by ECI Digital Archives,

5 468 DAVIES, KROUSE, TAUPO 2 The 2th International Conference ( xi on xfluidization i ) - New Horizons in Fluidization Engineering, Art. 56 [2007] i= 2 n VHI = (2) ( n ) and by considering the case for large n, it can be shown that : V V TOT HI 2 ( ρ ) (3) where n is the number of data points and ρ is the correlation between consecutive values in the data series. Decreasing estimates of (V TOT / V HI ) can thus be interpreted as decreasing correlation between successive data values; the asymptote is 0.5 provided the correlation is non-negative. In Figure 5 we have plotted (V TOTP / V HIP ) against (U / U mfσ ); in all cases, the sampling frequency was 50Hz. Clearly there is evidence of a changing degree of correlation, and significantly, for the bubbling bed region where U>U mf, (V TOTP / V HIP ) 5. A B C D E F Figure 4 Bed pressure drop (normalized) versus time for Mixture 4. σ p (Pa) indicates the scale of the ordinate; U is ms - A, σ p =0.72, U=0; B, σ p =.77, U=0.0076; C, σ p =3.65, U=0.04; D, σ p =7.5, U=0.022; E, σ p =3.38, U=0.033; F, σ p =59.8, U=0.052 According to Parseval s theorem, variance is the area under the spectral density. Previously when analyzing time series data from a particulate system, we have found the variance to be a sensitive parameter when the noise has a / f α type spectrum (7). Difficulties arise for α, as the variance becomes infinite and low frequencies dominate. A flexible mathematical model is provided by fractionally differenced series, which are used to model long-memory time series. The fractional differencing parameter, d, satisfies α=2d and is also related to the Hurst parameter (8) via d=h 0.5. We use the method of Geweke and Porter- Hudak (9) to estimate d, and this is plotted in Figure 6 for the Mixtures listed in Table. It is of interest to note that immediately below U mfσ, d corresponds to a / f noise spectrum, whereas above minimum fluidization the differencing parameter is small or in some cases essentially zero indicating little or no correlation between successive data points. Attractor Reconstruction particle size Here we investigate whether attractor reconstruction techniques can provide 4

6 FLUIDIZATION XII 469 additional information Davies et al.: not Particle contained Size Estimation in and the Monitoring variance, in a Bubbling and Fluid in Bed particular whether changes in the particle-size distribution can be detected. Specifically we follow van Ommen et al (0) who 00 found that the S-statistic of Diks et al () could be used to detect small 0 changes in the particlesize distribution. V TOT /V HI U/U mfσ Figure 5 Ratio of total variance to high frequency variance as a function of minimum fluidizing velocity measured by pressure fluctuations The S-statistic, S, is used to compare attractors reconstructed using the delay vectors of a single characteristic variable, for example, pressure. We calculate S using the Equations numbered, 3 and 7 in reference (0) applied to uncorrelated delay vectors, which are approximately independent. To obtain the necessary vectors from a given sequence x[] x[n], there must be a spacing, h, such that the sub-sequence with ith datum, x[+(i-)h], has zero autocorrelation. Then the m-dimensional delay vectors x[] x[n], where x[k]=(x[+(k-)(m+h)] x[km+(k-)h]), are uncorrelated and the number satisfies N (n+h)/(m+h). For a fixed number of delay vectors, N, we take the largest possible spacing to ensure the delay vectors are uncorrelated. This maximal spacing is given by n N Nm (4) As a check for residual correlation between the delay vectors, we estimate the standard deviation, σ S, of the S-statistic under the null hypothesis by bootstrap resampling from a suitable time-series model of the reference series. Specifically, we use a pth order autoregressive model, AR(p), which has the form x t p = a k = k x t k + ε t (5) The bootstrap procedure is as follows (i) estimate the AR(p) model parameters for the reference series using the Akaike Information Criterion to estimate the order (ii) simulate two independent series using the model parameters and residuals estimated in (i) (iii) calculate the S-statistic for the simulated series Published (iv) repeat by ECI Digital steps Archives, (ii)-(iii) times 5

7 470 DAVIES, KROUSE, TAUPO The 2th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 56 [2007].2 fractional differencing parameter d [-] U/U mfσ [-] Figure 6 Variation of d with (U/U mfσ ), data sampled at 50Hz The bootstrap estimate of the standard deviation, σ S, is the standard deviation of the values calculated in step (iii). When monitoring fluidized beds van Ommen et al (0) first normalize the pressure, P, to eliminate the main effects of superficial velocity, and define ( P P) y = (6) σ p To check for sensitivity to particle size, we compare the finest and coarsest sands, viz Very Fine and Coarse, at three values of U; one Table 2 Comparison value greater than the U mfσ for both materials, one lying of Coarse and Very between the U mfσ for both materials, and one value Fine base mixtures; smaller than the U mfσ for both materials which was also S-statistic and σ S smaller than the U mf P for both materials. U [m/s] S σ S We use the optimal parameter settings from Table 2 in reference (0) (i.e. m=20, band-width parameter is 0.5), and compare the delay vector distributions using the coarse sand as the reference. Each series has n=892 data points and N=200 uncorrelated delay vectors are used in calculating S. The results are given in Table 2 and are consistent with the trend observed by van Ommen et al (0), who found that the sensitivity of the S-statistic was greatest at low gas velocities. DISCUSSION The materials in Table have mean particle diameters ranging from ~00µm to ~70µm and a particle density of 2480kgm -3, and thus lie in Geldart Group B, but close to the A/B boundary. However, for all powders for which U mfσ was measured, (Uhttp://dc.engconfintl.org/fluidization_xii/56 mfσ / U mf P )>.6. In contrast to our results, the data reported by Puncochar et al (2) 6

8 FLUIDIZATION XII 47 and Wilkinson (2) Davies give et al.: Particle (U mfσ Size / UEstimation mf P ). and Puncochar Monitoring in a et Bubbling al (2) Fluid worked Bed with Geldart Group D or B/D materials and Wilkinson (2) used two samples of silica glass ballotini, with mean diameters 305µm and 87µm, and particle densities of 2550 kg m -3 and 2650 kg m -3 respectively, placing them well into Geldart Group B. Our measurements of the parameter we have termed U mfσ thus record the onset of bubbling rather than minimum fluidization, and appear to provide a sensitive measure of Geldart Group A behaviour; more apt terminology would be U mbσ. With reference to the Geldart diagram for a constant material density, and moving away from the A/B boundary as particle size increases we would expect to see more typical Group B behaviour, viz U mf P =U mbσ. Using the data in Table, a plot of (U mfσ U mf P ) / U mf P against (U mf P ) 0.5 decreases linearly with increasing U mf P ; and if the observed trend were to continue, (U mfσ / U mf P )= at U mf P 0.042, equivalent to a particle diameter of ~230µm for this material. Our analysis using the ratio of total variance to high speed variance, (V TOTP / V HIP ) has highlighted the utility and sensitivity of this parameter for regime identification. The form of Figure 5 suggests that there is an abrupt change in the bed, interpreted as the onset of bubbling, and this is consistent with Figure 6 where the plot of the fractional differencing parameter d indicates little correlation between successive measurements for (U / U mfσ )>~. Likewise at low superficial velocities, (U / U mfσ )<~, the S-statistic provides additional information not contained in the variance that can be used to distinguish different material samples. CONCLUSIONS Pressure fluctuation data for six silica sand mixtures prepared from two reference samples with mean diameters of ~00µm and ~70µm have been analysed over superficial velocities spanning the range 0 (U / U mf )<~5. The characteristic system parameters used were the ratio of total variance to high speed variance, (V TOTP / V HIP ); the fractional differencing parameter, d, which is defined as the Hurst exponent minus 0.5; and the S-Statistic. The statistic (V TOTP /V HIP ) is simple to calculate and is a powerful indicator of change within the systems we have considered. The fractional differencing parameter highlights changes in bed structure over the whole test range of superficial velocities from 0 to (U / U mf P ) 5; in bubbling beds where (U / U mbσ ), d takes values between ~0 and ~0.2 which indicates only short term correlation between data points. Additionally, the S-statistic provides a sensitive test for distinguishing different test materials at low superficial velocities. NOTATION a autoregressive model parameter [-] AR(p) autoregressive time-series model of order p d fractional differencing parameter defined as (H-0.5) [-] f frequency [Hz] h spacing between data value indices for negligible correlation [-] H Hurst parameter [-] m dimension of delay vectors [-] n number of data points [-] N number of m-dimensional delay vectors [-] P pressure [Pa] Published by ECI Digital Archives, 2007 P mean pressure [Pa] 7

9 472 DAVIES, KROUSE, TAUPO Re p The 2th International particle Conference Reynolds on Fluidization number - New [-] Horizons in Fluidization Engineering, Art. 56 [2007] S S-statistic [-] U superficial gas velocity [ms - ] U mbσ estimate of minimum bubbling velocity using σ p method [ms - ] U mf minimum fluidizing velocity [ms - ] U mf P estimate of U mf using pressure drop method [ms - ] U mfσ estimate of U mf using σ p method [ms - ] V HI, V HIP high frequency variance [units of data variable] 2 V TOT, V TOTP data variance [units of data variable] 2 x data value [units of data variable] x delay vector [units of data variable] y normalized pressure fluctuation [-] ε t additive component of noise at time index t [units of data variable] α frequency spectrum exponent [-] ρ correlation between successive data points [-] σ p, σ S standard deviation [units of data variable] i, k, label for data point [-] t, t-k time index [-] subscript p, pressure; S, S-Statistic REFERENCES. L.T. Fan, S. Hiraoka, and S.H. Shin, AIChE Journal, 30 (984) M. Puncochar, J. Drahos, J. Cermak, and K. Selucky, Chem. Eng. Commun., 35 (985) C. E. Davies and K. Fenton, IPENZ Transactions, 24, /EMCh (997) Clive E. Davies, Alison Carroll and Rory Flemmer, Particle size monitoring in a fluidized bed using pressure fluctuations, submitted to Powder Technology (2006). 5. D. Kunii and O. Levenspeil, Fluidization Engineering, Robert E. Kreiger Publishing Co. Inc., New York (977) J. von Neumann, R.H. Kent, H.R. Bellinson and B.I. Hart, Ann. Math. Stat. 2 (94) Donal P. Krouse and Clive E. Davies, Journal of Chemical Engineering of Japan, 37, 2 (2004) Edgar E. Peters, Chaos and Order in the Capital Markets, John Wiley and sons, New York (996). 9. J. Geweke and S. Porter-Hudak, Journal of Time Series Analysis, 4 (983) J.R. van Ommen, M.-O. Coppens and C.M. van den Bleek, AIChE Journal, 46,, (2000) C. Diks, W.R. van Zwet, F. Takens and J. DeGoede, Phys. Rev. E, 53 (996) Derek Wilkinson, The Canadian Journal of Chemical Engineering, 73, (995)